Adjustable constant current network



1966 E. R. ANDERSON ETA}... 32%,552

ADJUSTABLE CONSTANT CURRENT NETWORK Filed Aug. 23, 1963 {5 Sheets-Sheet 1 INVENTOR5 i'MMirrZfl/wiiiom BY 0 mm 1d. f bez 1966 E. R. ANDERSON ETAL 3,264,552

ADJUSTABLE CONSTANT CURRENT NETWORK 5 Sheets-Sheet 2 Filed Aug. 23, 1963 5. w W4 m m j WFM W14 Z 4 WW I;

2, 5 E. R. ANDERSON ETAL 352M552 ADJUSTABLE CONSTANT CURRENT NETWQRK Filed Aug. 23, 1965 3 Sheets-$heet 3 5 (Av/TYRE 1NVENTOR5 iM/wirr Rflwazkiam ff/#2455 4/. PM:

mm, mm M M 4 0w! United States Patent 3,264,552 ADJUSTABLE CONSTANT CURRENT NETWORK Emmett R. Anderson and Charles W. Park, Oakland, Calif, assignors to Temescal Metallurgical Corporation, Berkeley, Calif., a corporation of California Filed Aug. 23, 1963, Ser. No. 304,184 7 Claims. (Cl. 32376) This invention relates to constant current networks in general and is particularly directed to arr-improved constant current network for providing a 0 to 100% range of adjustable constant load currents without undue stress of the network circuit elements.

Various constant current networks are known wherein a constant output load current is produced which is a function of the input voltage applied to the network. Accordingly, through variation of the input voltage, the out put load current may be correspondingly adjusted to different constant current values, each of which remains constant irrespective of load fluctuations. Such adjustable constant current networks may be employed to great advantage in certain constant current systems as a mechanism for readily controlling the current, and power, delivered to a load. For example, in high-vacuum electron beam furnaces constant current systems are frequently employed to control the amount of electron beam bombardment current and, therefore, power delivered to melt stock in a crucible or the like. In such a system, a constant current network produces constant alternating current, which in turn is rectified by a high voltage rectifier to constant direct current. The direct current output of the rectifier is supplied in series to the bombarding electron beam emanating from an electron gun, or equivalent source, and impinging the melt stock, the beam thus constituting the load of the constant current system. By appropriately controlling the filament temperature of the electron gun, the beam load resistance can be changed so that a constant voltage can be maintained across the electron beam load as the load current flowing in the constant current system is adjusted over a wide range of constant current values. Since the load voltage is maintained constant while the load current is varied, the amount of power delivered to the load is correspondingly precisely determined by the amount of load current flowing in the constant current system. It is thus particularly desirable that the current output of the constant current system be readily precisely adjustable over a wide range. To this end a continuously adjustable input voltage is preferably applied to the constant current network of the system to facilitate continuous adjustment of the current over a range of 0 to 100% rated load current. Also, in connection with A.C. welders and in various other applications, it is desirable that the output current of a constant current network be continuously adjustable.

Although vraious types of constant current networks may be employed in the foregoing, and other constant current applications, various types of monocyclic constant current networks are preferred. Monocyclic networks include inductors and capacitors connected to provide circuits at resonance with alternating current input voltage applied thereto. More particularly, where three-phase input voltages are employed, the monocyclic networks are commonly of a basic delta mesh configuration. The three legs of the delta mesh are each defined by an inductor and a capacitor connected in series and tuned to series resonance with the three-phase input voltage. In addition, the legs are connected such that the inductors and capacitors are disposed in alternate succession around the mesh. The three phases of the input voltage are then respectively applied to the junctures between the three 3,264,552 Patented August 2, 1966 from various points between the inductors and capacitors which depend upon the particular circuit design. In one particular advantageous monocyclic network, which is herein termed diametric monocyclic constant current nettive to the electron beam vacuum furnace, the load is subject to frequent interruptions which give rise to transients in the constant current network. The diametric constant current network is accordingly advantageous from the standpoint of the minimized stressing of the network circuit elements under transient conditions. In addition, the capacitance of the capacitors required in the diametric constant current network is less than that required in other monocyclic networks designed for operation at the same input voltage. However, although the conventional diametric monocyclic network possesses the foregoing advantageous characteristics while providing an adjustable constant current output in response to variation of input voltage, such circuit still leaves much to be desired in adjustable current applications, particularly where the current is to be adjusted over a complete range of from 0 to 100%.

More particularly, as the input voltage to the diametric network is decreased to, in turn, decrease the output current, the voltages across the reactive elements of the network increase and become untenably large at the lower end of the output current range. Accordingly, it becomes uneconomical to employ reactive elements having the re quired kva. ratings to withstand the substantial voltages existing thereacross under low input voltage, low output current conditions. Furthermore, inductive reactors are commonly employed as the inductive elements of the net work, and the voltages thereacross increase far above the rated maximum as the current is adjusted downwardly through the operating range. Such inductive reactors f are consequently forced into saturation if the network is operated over the full extent of the desired. 0 to- 100% output current range; the capacitors are likewise overlegs and a three-phase constant output current is derivable stressed.

monocyclic network are preserved while the disadvantages thereof for variable input voltage, adjustable output current operation are overcome.

Another object of the invention is the provision of an adjustable constant current network which is capable of delivering output currents over a range of from O to without unduly stressing the circuit elements of the network, even at the lower end of the output current range.

It is still another object of the invention to provide an adjustable monocyclic constant current network of the class described in which voltage rises under transient conditions are not excessive.

It is a further object of the invention to provide an adjustable monocyclic constant current network wherein the voltages across the reactive elements thereof are comparatively low over the. entire range of adjustable output current such that reactive elements of economically attractive size may be employed in the network.

Additional objects and advantages of this invention will become apparent upon consideration of the following detailed description of several particular preferred embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a schematic circuit diagram of'a conventional diametric monocyclic constant current network adapted to continuously adjust output current service;

FIGURE 2 is a vector diagram of voltages existing in the conventional diametric circuit of FIGURE 1 at several operating points in its range of adjustable output current, and illustrating particularly the substantial increase in voltage across the reactive elements of the network at reduced values of output current and fixed load voltage;

FIGURE 3 is a schematic circuit diagram of an improved adjustable constant current monocyclic network in accordance with the present invention, arranged to provide a three-phase adjustable current output from a variable three-phase voltage input;

FIGURE 4 is a vector of voltages existing in the improved circuit of FIGURE 3 at various operating points in its adjustable output current range comparable to those depicted in the vector diagram of FIGURE 2 for the conventional diametric network of FIGURE 1;

FIGURE 5 is a schematic circuit diagram of a modified form of three-phase adjustable constant current monocyclic network in accordance with the invention;

FIGURE 6 is a vector diagram of voltages existing in the circuit of FIGURE 5 at various points in its adjustable output current range comparable to those depicted in the vector diagrams of FIGURES 2 and 4; and

FIGURE 7 is a schematic circuit diagram of another modified form of adjustable constant current network in accordance with the present invention, arranged to provide an adjustable single-phase output current from a variable three-phase voltage input.

It will be of assistance in understanding the adjustable constant current network of the present invention and appreciating the advantages thereof to consider at the outset of conventional diametric monocyclic constant current network and various circumstances which arise during adjustable current operation thereof with varied values of input voltage. With reference to FIGURE 1, such a conventional diametric monocyclic network is illustrated as including three resonant circuit legs 11, 12,

1 13 connected at junctures 14, 16, 17 in a delta mesh convoltage mechanisms are employed to couple the junctures to three-phase supply lines, or terminals A, B, C. These reference characters applied to the input terminals likewise are indicative of the respective phases of voltage existing thereat in accordance with standard three-phase circuit convention.

Although a variety of variable voltage mechanisms may be employed in the conventional diametric monocyclic network, as depicted in the figure, variable transformers, preferably auto-transformers 21, are best employed for this purpose. More particularly, the respective windings 22 of the variable auto-transformers 21 are in Y-connection with the three-phase terminals A, B, C, while continuously variable voltage taps or swingers 23 of the variable transformers are connected to the respective junctures between the legs. Thus, upon adjustment of the taps 23 of the variable transformers 21, the amplitudes of the three-phase voltages applied to the respective junctures between the legs of the mesh are correspondingly varied.

Constant current output from the network is utilized by means of transformers 24 having primary windings 26 each connected between one of the leg junctures 14, 16, 17 and the juncture 27 between the inductor 18 and the capacitor 19 of the diametrically opposite leg. For example, the primary winding 26 of one of the transformers is connected between juncture 16 and juncture 27 of diametrically opposite leg 11. The secondary windings 28 of transformers 24 are, in turn, in Y-connection with three-phase constant current output terminals 29.

With the conventional diametric monocyclic network just described, constant current is delivered from output terminals 29 upon the connection of a load thereto, and the amplitude of such current is a function of the input voltage applied to the three corners, or leg junctures, of the mesh. Accordingly, upon adjustment of the variable transformers 21 to apply input voltages to the network ranging from 0 to of the three-phase line voltage supplied to the terminals A, B, C, the output current from terminals 29 is correspondingly varied over a range of constant current values from 0 to 100%. Unfortunately, as the input voltage and output current are adjusted downwardly from 100%, voltages across the inductors 18 and capacitors 19 increase substantially, and in the lower end of the range the voltages across these reactive elements are of the order of or more, than the voltage values for 100% output current. Of more importance, the kva. of the reactive elements increases as the square of the voltage and, hence, becomes untenably large. Inasmuch as the size and cost of the reactive elements is determined by the kva., it will be seen that it is I not economically feasible to design a conventional monocyclic network for operation over a wide range of input voltage.

The substantial increases in voltage across the inductors 18 and capacitors 19 of the conventional diametric monocyclic network will be more fully appreciated upon reference to the vector diagram of FIGURE 2 which depicts various voltages existing in the conventional diametric circuit at various operating points in the output current range for a fixed load Voltage. More particularly, vectors A C C B and A B represent 100% input voltage applied to the diametric constant current network; in other Words, the three-phase, lineto-line voltages applied to terminals A, B and C. Similiarly, vectors A N, B N and C N represent the corresponding line to neutral voltages applied to the leg junctures of the mesh from the variable transformers 21. It will be noted that the 100% input voltage vectors give rise to a fixed voltage component C X across the primary winding 26 of each output transformer 24. In addition to the fixed voltage vector component, there is an imaginary voltage component X D in series therewith, determined by the load resistance and the phase rotation. The imaginary voltage component is commonly made equal to the fixed component for 100% load resistance variation. More particularly, a diametric constant current network is commonly designed such that the voltages A D and B D across, respectively, the inductors 18 and capacitors 19 produce the resultant imaginary voltage component X D equal to the fixed line component C X Hence, for 100% input voltage, the voltage vectors A D and B D across the reactive elements of the circuit are equal to the three-phase, line-to-line voltages A C C B and A B It should also be noted that the current through the primary windings 26 of output transformer 24 is the vector difference between the current flowing in inductors 18 and the current flowing in capacitors 19.

Now, assume the varible transformers 21 are adjusted to reduce the input voltage to the network to 75% of line voltage. In FIGURE 2, the vectors A C C 13 and A 8 represent the corresponding 75 three-phase input voltages applied to the network, and these voltages are productive of a resultant fixed line voltage component C X This fixed component determines a corresponding 75% constant output current from terminals 29. It is particularly important to note that the load resistance is adjusted so that the sum of the fixed voltage component and the imaginary voltage component is a constant irrespective of the amount of input voltage applied to the network. Hence, the sum of the fixed component C X and the corresponding imaginary component X D for 75 input voltage isequal to the sum of the fixed component C X and the imaginary component X 13 for 100% input voltage. Inasmuch as the fixed component has decreased, the imaginary component has increased for the reduced input voltage of 75%. Correspondingly, the inductor voltage A D and capacitor voltage B D for 75% input voltage have increased over the corresponding reactive voltages existing for 100% input voltage. In fact, the voltages A D and B D are substantially 115% of the 100% reactive voltages A 13 and B D Such increased reactive volt-ages increase the current through the inductors and capacitors and the kva. rating on the inductors and capacitors has increased to 132%. Similarly, as the input voltages are reduced further and the fixed component of voltage is decreased, the imaginary component of voltage increases as do the voltages across the reactive elements. In the vector diagram, the characters bearing the subscript 3 depict voltage relationships for a reduction to 50% input voltage whereas the characters bearing the subscript 4 depict voltage conditions when the input voltage is reduced to 25%. It will be found that for the 25% input voltage case (and, therefore, 25% output current), the inductor and capacitor voltages A D and B D respectively, are 152% and the kva. is increased to 230% compared to the voltages existing under 100% input voltage and output current conditions. One or the other of the reactive elements is even further stressed when the power factor of the load deviates from the unity power factor condition depicted in FIGURE 2. In this regard, the arcs G are loci of constant voltage variable power factor load for the various input voltages discussed hereinbefore, the directions of power factor lag and lead being as indicated in the figure. As the power factor varies from unity, the points D D D D swing along the arcs. Accordingly, for a lagging power factor load, the voltage across the capacitors 1% (H 13 B D etc.) is increased while the voltage across the inductors 18 (A 13 A D etc.) is decreased from the voltage value for unity power factor. Conversely, for a leading power factor the capacitor vol-tage decreases while the inductor voltage increases.

It is thus readily apparent that in order to cover a full range of from 0 to 100% output current by adjusting the input voltages through .a similar range, the inductors and capacitors of a conventional diametric monocyclic constant current network must be of an untenably large size in order to withstand the substantial reactive voltages existing in the circuit at the lower end of the output current range. Moreover, the use of inductive reactors as the inductive elements of the constant current network is substantially precluded where such a network is used for adjustable current service inasmuch as these inductive reactors are commonly designed to be linear up to only about 125% of their full load operating voltage. Any increase in voltage beyond this amount causes the reactors to become non-linear and go into saturation. Since conditions would prevail when a conventional network was operated under conditions of about 65% output current and lower, it will be appreciated that it is not economical to design inductive reactors for the voltages encountered in, for example, the output current condition.

The foregoing difiiculties of conventional diametric monocyclic constant current networks when employed to produce an adjustable output current are overcome in accordance with the present invention by a circuit of the type depicted in FIGURE 3. As shown therein, an improved monocyclic network is provided which is, in basic respects, similar to the diametric network of FIGURE 1, but which embodies improved connections in accordance with the present invention. In the circuit of FIG- URE 3, components which are comparable to those of the circuit of FIGURE 1 bear corresponding reference characters, and it is to be noted that except for the primary windings 26 of the output transformers 24, the

circuit components are connected in identical fashion as in the circuit of FIGURE 1. More particularly, the primary windings 26 are still connected at one end to the junctures 27 between the inductors 18 and capacitors 19 in the respective legs 11, 12, 13; however, the other ends of the primary windings are no longer connected to the diametrically opposite junctures 14, 16, 17 between the legs. Instead, in accordance with the present invention, the opposite end of each output transformer primary winding is coupled to a source of fixed reference voltage of a phase intermediate the two phases energizing the opposite ends of the particular leg to which the first end of the transformer primary is connected. In other words, the transformer winding, which is connected at one end to the juncture 27 of leg 12, for example, is connected at its opposite end to a fixed reference voltage having phase C, leg 12 being energized at its opposite ends with voltages having phases A and B. The fixed reference voltages moreover are preferably the line voltages .applied to terminals A, B and C; and hence, the transspecific components of the improved circuit of FIGURE 3, particularly as to the types of transformers which may be utilized. The variable transformers 21, stated hereinbefore as being preferably variable auto-transformers, may alternatively be induction voltage regulators, isolated winding variable transformers, or the like. Whatever the transformer type employed, the transformers 21 may be provided as three-single-phase units having their voltage varying swingers or wipers ganged, or as a single three-phase variable transformerunit. The output transformers 24 may likewise be provided as three singlephase units or as a single three-phase unit. However, it

should be noted that the single three-phase unit must have the leads to the primary windings 26 separated in order to provide six separate leads for facilitating the diametric connections.

Considering now the effect of connecting the ends of the transformer primaries to fixed reference voltages in accordance with the present invention, instead of employing the customary connection to the diagonally opposite juncture of the network mesh, reference is made to FIGURE 4, which depicts a vector diagram of the voltages existing in the improved circuit of the instant invention under varied conditions of input voltage. For 100% input voltage, it will be noted that the voltages existing in the circuit are identical to those depicted in FIGURE 2 for a conventional diametric network at the 100% operating point. However, when the circuit of the present invention is operated at reduced input voltage in order to provide a reduced constant current output, an additional component of voltage is provided which is proportional to the difference between the full line voltage, or other fixed reference voltage, applied to the output transformer primaries and the input voltage applied to the junctures 14, 16, 17 between the legs of the mesh. Such an added voltage component provides a portion of the voltage increase, normally contained in the imaginary voltage component existing in a conventional diametric network, to maintain the sum of the fixed line voltage component and imaginary voltage component a constant. More particularly, it will be noted, for example, that for 75% input voltages A C C 3 and A B a fixed voltage component C X is provided and an imaginary voltage X D associated with the reactive elements of the circuit is provided to produce the load voltage C D. It will be thus appreciated that for sponding reactive voltages are 152% 75% input voltages, the fixed component isstill taken from point C rather than from point C as occurs with the conventional network of FIGURE 1. The imaginary component X D is thus reduced by an amount C C from the imaginary component X D of the conventional network under comparable 75% input voltage conditions. As a result, the voltages A D and B D existing across the inductors 18 and capacitors 19 of the network to produce the resultant imaginary component X D for the 75% operating point are substantially reduced. Similarly at further decreased input voltages, the added voltage component supplied by the fixed reference voltage is proportionately increased to correspondingly minimize the expansion of the imaginary voltage component, and

therefore, the voltages existing across the reactive elements of the network. In fact, at the 25% operating point, the voltages A D and 13 D existing across respectively the inductors 18 and capacitors 19 are only 109% (kva. 119%) of the corresponding voltages for 100% input voltage energization. It will be thus appreciated that the reactive voltages existing in the network of the present invention, even at the lower end of the operating range, are manifestly reduced, it being hereinbefore noted that under conditions of 25% input voltage applied to a conventional diametric network, the corre- (kva. 230%). Although the foregoing voltages may be increased to some extent when the power factor deviates from unity and point D swings along the variable power factor locus G, such voltages are still manifestly less than the comparable voltages for a conventional network. Thus, reactive elements of economically attractive size may be readily employed in the improved network of FIGURE 3 by virtue of the substantially reduced reactive voltages existing therein. Moreover, inductive reactors of conventional design may be employed as the inductors 18 without detriment from saturation inasmuch as the reactive voltages existing in the circuit, even under conditions of voltage input and current output, do not exceed 116%, and these conventional reactors are commonly designed to be linear up to 125% of rated voltage. In addition, the comparatively superior behavior of a conventional diametric constant current network under transient conditions is not detrimented by the provision of the fixed reference voltages in accordance with the present invention, the superior transient capabilities of a conventional diametric network being preserved in the network of the present invention.

Considering now a modified form of the invention and referring to FIGURE 5, there will be seen to be provided an adjustable constant current network which in general respects is quite similar to the circuit of FIGURE 3 discussed hereinbefore. In the circuit of FIGURE 5, components which are similar to those of the circuit of FIG- URE 3 bear corresponding reference characters, and in this regard it is to be noted that except for the means provided to supply three-phase energization to the circuit, the circuit components are connected in identical fashion as in the circuit of FIGURE 3. More particularly, the output transformers 24 are provided with their primary windings 26 respectively connected at one of their ends to the three-phase input terminals A, B, C and at their other ends to the junctures 27 between the inductors 18 t and capacitors 19 of the legs 13, 11 and 12, respectively. In addition, the secondary windings 28 of the output transformers are respectively connected to three-phase constant current output terminals 29. To this extent, the

. respective circuits of FIGURES 3 and are identical,

each output transformer primary winding being con nected at one end to a source of fixed three-phase reference voltage of a phase intermediate the two phases energizing the opposite ends of the particular leg to which the first end of the primary is connected However, whereas the circuit of FIGURE 3 is arranged for variable input voltage energization from Y-connected variable transformers, the circuit of FIGURE 5 utilizes delta-connected variable transformers in the energization thereof. More particularly, it is to be noted that variable input transformers 31 are provided with windings 32 respectively connected between terminals A and B, B and C, and C and A, to thus define a delta connection. Although the transformers 31 may be induction voltage regulators, isolated core variable transformers, etc., they are preferably continuously variable auto-transformers as depicted in the figure. It is of importance to note that input transformers 31, instead of being provided with single swingers or wipers as in the instance of FIGURE 3, are each provided with a pair of voltage varying wipers 33, 34 which are ganged to provide continuously variable voltages which are equal for the respective phases connected to the opposite ends of the winding at any given setting of the wipers. The wipers 33, 34 of the input transformers are connected to the opposite ends of the respective legs 11, 12, 13 of the monocyclic delta circuit mesh. It will be thus appreciated that the foregoing connections of the transformers 31 facilitate variable threephase voltage energization of the improved monocyclic network from a delta connection. Here again, upon varying the input voltage energization through manipulation of the wipers 33, 34 the constant current output obtainable from terminals 29 is correspondingly varied in magnitude. Of more importance, the direct connections of the junctures 27 to the line terminals A, B, C results in minimized stressiing of the reactive elements of the network (inductors 18 and capacitors 19) in a similar manner to that produced by the circuit of FIGURE 3. However, in the present instance the minimization effect is even more pronounced, as will be evident upon consideration of the vector diagram for such circuit as depicted in FIGURE 6.

Referring to FIGURE 6, it is to be noted that, for 100% input voltage, the voltages existing in the circuit of FIG- URE 5 are identical to those existing in the circuits of FIGURES 1 and 3 at the 100% operating point. When the input voltage to the network of FIGURE 5 is reduced, however, the reactive voltages are likewise reduced rather than increased. More particularly, for 100% input voltages A C C B and A B a fixed voltage component C X and an imaginary voltage component XD associated with the reactive elements of the circuit are provided to produce the load voltage C D. Now, for 75% input voltages A C C B and A B the load voltage is again C D, and provided by the sum of the fixed component C X and'imaginary component XD. Likewise, with any other percent input voltage, the load voltage is the sum of the fixed component C X and imaginary component XD. Thus, the locus of the reactive voltages A D, B D; A D, B D; etc., as the input voltages are decreased, is along the line-to-line voltage A 13 rather than along the line-to-neutral voltages A N, B N as occurs for the network of FIGURE 3. Consequently, the reactive voltages at reduced input voltage are less than what they are at the 100% operating point. For purposes of comparison, it should be noted that the reactive voltages A D and RD across inductors 18 and capacitors 19 are 87.5% (kva. 76.6%) of the corresponding voltages for 100% input voltage energization. Here again the voltages may be increased to some extent when the power factor deviates from unity, and point D swings along the variable power factor locus G. However, the voltages are still manifestly less than the comparable voltages for the conventional network of FIGURE 3, and even for the improved network of FIGURE 5. It will be thus appreciated that for the network of FIGURE 5, the circuit Q justable three-phase constant output current from a variable three-phase input voltage, the principles of the invention apply equally as well to circuits adapted to provide a single-phase output current. In this regard, a circuit in accordance with the present invention adapted to provide an adjustable single-phase output current is depicted in FIGURE 7. Such circuit includes a pair of variable inductive reactors 31 or equivalent voltage varying means, which have windings 32 serially connected between three-phase input terminals A and B. Continuously variable voltage taps 33 of the variable autotransformer are respectively connected to opposite ends of a reactive circuit path consisting of an inductor 34 and capacitor 36 connected in series and tuned to series resonance with the frequency of voltage applied to the terminals A and B. In addition, an output transformer 37 or the like, is provided with -a primary winding 38 connected at one end to the juncture 39 between the inductor 34 and capacitor 36, and connected at the other end to a three-phase input terminal C supplied by voltage of the phase which is intermediate those supplied to terminals A and B. A secondary winding 41 of the output transformer 37 is in turn connected at opposite ends to output terminals 42, from which constant current may be supplied to a load. As the voltage applied to the opposite ends of the reactive circuit path defined by inductor 34 and capacitor 36 is varied by means of the variable reactors 31, the output current from terminals 42 varies correspondingly. Moreover, by virtue of the connection of the transformer primary 38 to the fixed reference voltage provided at terminal C, the voltages existing across the inductor 34 and capacitor 36 are minimized over the entire range of operating current, in a manner analogous to that described relative to the circuit of FIGURE 3. As a result inductors and capacitors may be employed in the circuit which are of relatively small rating, and still the output current may be adjusted over the entire range of from to 100% Although the present invention has been described hereinbefore with respect to several specific embodiments, it will be appreciated that various changes and modifications may be made therein without departing from the true spirit and scope of the invention. For example, the invention may be employed with similar advantages with power flow in the reverse direction for purposes of transforming a variable current source into a variable voltage. In this case, the phase rotation would be reversed from a given rotation for the described employment of the invention in producing a variable constant current output.

We claim:

1. An adjustable constant current network comp-rising at least one current path defined by a capacitor and an inductor connected in series and tuned to series resonance with a predetermined frequency, a transformer including a primary winding with a first end connected to the juncture between said capacitor and inductor of each current path and including a secondary winding adapted for connection to a load, variable voltage means coupling variable amplitude first and second phases of a three-phase sequence of voltages having said predetermined frequency to opposite ends of each said current path, and means coupling the third phase of said voltages to the second end of the corresponding primary winding with a fixed amplitude equal to at least the maximum amplitude of said variable amplitude first and second phases of voltages.

2. An adjustable constant current network comprising three legs each defined by an inductor and capacitor connected in series and tuned to series resonance with a predetermined frequency, said legs connected in delta with said inductors and capacitors disposed in alternate succession, a three-phase voltage source including lines carrying voltage at said predetermined frequency and in respective three-phase sequence, variable voltage means coupling each said lines to a different one of the junctures between said legs to supply variable amplitude three-phase volt- 1% ages thereto, three output transformers respectively having primary windings with first ends connected each to a different one of the junctures between said inductors and capacitors defining said legs, and means coupling threephase voltages of fixed amplitude to second ends of said primary windings with the same phase as that of the variable amplitude three-phase voltages supplied to the junctures between legs diametrically opposite the junctures between said inductors and capacitors to which the first ends of said primary windings are connected, said voltages of fixed amplitude being equal to at least the maximum amplitude of said variable amplitude three-phase voltages.

3. An adjustable constant current network according to claim 2, further defined :by said means coupling threephase voltages of fixed amplitude to second ends of said primary windings comprising conductors connecting said second ends of said windings directly to said lines coupled to said junctures between said legs diametrically opposite said junctures between the inductors and capacitors to which the first ends of the respective windings are connected.

4. An adjustable constant current network comprising three legs each defined by an inductor and a capacitor connected in series and tuned to series resonance with a predetermined frequency, said legs connected in delta with said inductors and capacitors in alternate succession, variable transformer means including windings Y- connected to a three-phase voltage source and variable voltage output terminals respectively connected to the junctures between said legs, and output transformer means respectively having primary windings respectively connected between the junctures between said capacitors and inductors defining said legs and the ends of said windings of said variable transformer means connected to said voltage source and associated with the corresponding diametrically opposite junctures between said legs.

5. An adjustable constant current network comprising first, second and third three-phase input terminals adapted for connection to a three-phase voltage source of predetermined frequency, an inductor and capacitor connected in series and tuned to series resonance with said predetermined frequency, a pair of variable inductive reactors having series-connected variable inductors with their free ends connected to said first and second input terminals and having variable output terminals connected to the opposite ends of the series combination of said inductor and capacitor, and an output transformer having a winding connected between said third input terminal and the juncture between said inductor and capacitor.

6, An adjustable constant current network comprising three legs each defined by an inductor and a capacitor connected in series and tuned to series resonance with a predetermined frequency, said legs coupled in delta with said inductors and capacitors in alternate succession, variable transformer means including windings Y-connected to a three-phase voltage source and variable voltage :output terminals coupled to opposite ends of said legs to energize same with variable three-phase voltage, and a plurality of output transformers respectively having primary windings connected between the junctures between said capacitors and inductors defining said legs and the ends of said windings of said transformer means connected to said voltage source and associated with the corresponding diametrically opposite junctures between said legs.

7. An adjustable constant current network comprising variable transformer means having windings A-connected between three-phase input voltage terminals and a pair of ganged variable voltage wipers associated with each said windings, an inductor and capacitor series connected between the wipers associated with each winding and tuned to series resonance with the frequency of three-phase voltage applied to said input terminals, said inductors and capacitors thereby coupled through said windings to de- 1 1 12 fine a A circuit mesh said inductors and capacitors dis- I References Cited by the Examiner posed in alternate succession about said mesh, and output UNITED STATES PATENTS transformer means having windings respectively connected between the junctures between said inductors and $3333; capacitors and said input terminals, each said wmdmg of 5 2,957,123 10/1960 Rose 323 76 said output transformer means connected to the terminal having a phase intermediate the phases of the terminals JOHN COUCH Primary Examiner connected to the inductor and capacitor Whose juncture is connected to the opposite end of said winding of said LLOYD MCCOLLUM Exammer' output transformer means. 10 A. D. PELLINEN, Assistant Examiner. 

1. AN ADJUSTABLE CONSTANT CURRENT NETWORK COMPRISING AT LEAST ONE CURRENT PATH DEFINED BY A CAPACITOR AND AN INDUCTOR CONNECTED IN SERIES AND TUNED TO SERIES RESONANCE WITH A PREDETERMINED FREQUENCY, A TRANSFORMER INCLUDING A PRIMARY WINDING WITH A FIRST END CONNECTED TO THE JUNCTURE BETWEEN SAID CAPACITOR AND INDUCTOR OF EACH CURRENT PATH AND INCLUDING A SECONDARY WINDING ADAPTED FOR CONNECTION TO A LOAD, VARIABLE VOLTAGE MEANS COUPLING VARIABLE AMPLITUDE FIRST AND SECOND PHASES OF A THREE-PHASE SEQUENCE OF VOLTAGES HAVING SAID PREDETERMINED FREQUENCY TO OPPOSITE ENDS OF EACH SAID CURRENT PATH, AND MEANS COUPLING THE THIRD PHASE OF SAID VOLTAGES TO THE SECOND END OF THE CORRESPONDING PRIMARY WINDING WITH A FIXED AMPLITUDE EQUAL TO AT LEAST THE MAXIMUM AMPLITUDE OF SAID VARIABLE AMPLITUDE FIRST AND SECOND PHASE OF VOLTAGES. 